370 OUTLINES OF CHORDATE DEVELOPMENT 



minimum of parental protection and care. Others, such as 

 the kitten or young rabbit, are born in a much less advanced 

 stage and, with unopened eyes and uncoordinated movements, 

 remain almost helpless for several days. Some of the marsu- 

 pial Mammals (Metatheria) are quite remarkable in that the 

 young are born after a very brief period of gestation (about 

 eight days in the opossum (Didelphys) and in Dasyurus), at a 

 relatively very early stage of development, not able even to 

 perform the simple act of sucking. In these forms there are 

 special adaptations to this condition, and the young are at 

 once transferred to a special external cavity of the mother, the 

 marsupium or pouch, where development proceeds. 



In no case is the young Mammal entirely independent of the 

 maternal organism for some time after parturition, for there 

 follows the second general period in the life-history, that of 

 lactation, during which the young organism is wholly or partly 

 dependent for its nourishment upon the mammary secretion 

 of the mother'. During this period development continues, of 

 course, but at a slower rate, and toward its close there is a 

 gradual transition to conditions of complete independence, 

 save that some degree of parental care may be exercised for a 

 . time longer. The duration of the period of lactation is variable, 

 even in a given species. 



Two other periods in the life-cycle of the Mammal need only 

 to be mentioned; these are the period of adolescence, during 

 which growth and development continue at a still slower rate, 

 and the period of adult life or sexual maturity. The transition 

 between these periods is often marked by a series of structural 

 and physiological alterations in characteristics other than those 

 of the reproductive system. 



Embryologically as well as morphologically, the Mammalia 

 present many similarities to the Sauropsida. The mammalian 

 ovum is nearly yolkless, and yet in its development it exhibits 

 many of the phenomena of yolk-influence characteristics of 

 eggs of the extreme telolecithal (meroblastic) type such as 

 the formation of a modified germ disc, of a (yolkless) yolk-sac, 

 and other less striking characteristics. While the mammalian 



